EDS

The EDS detector (as a component of an EDX Analyzer System) is a device to analyze x-ray photons emitted from a sample. The photons are generated when a sample is exposed to an electron or x-ray beam, such as in a Scanning Electron Microscope (SEM) or X-Ray Fluorescence (XRF) instrument.

The EDS detector is a self contained vacuum system (called a cryostat) with cryogenic pumping created by Liquid Nitrogen cooling or alternative methods such as peltier cooling.

The principal elements of an EDS detector are the entrance window, the Si (Li) crystal, a Field Effect Transistor (FET) and pre-amplifier. The entrance window allows photons to enter the detector, while maintaining the vacuum integrity of the cryostat. The critical front-end components, the crystal and FET are mounted on a cold finger within the detector cryostat, while the pre-amplifier is mounted on the exterior of the cryostat.

Entrance Window

The entrance window must maintain the integrity of the vacuum system and also allow transmission of the photons to the crystal. The two primary window materials are Beryllium or a polymer. The Beryllium windows are strong but only allow efficient transmission of photons greater than 1 KeV (Na). The polymer windows are very fragile and require a support grid and allow photon transmission down to 100 ev (B or even Be).

 

Crystal

The crystal starts as a moderately-doped p-type Si. Li is allowed to diffuse ("drift") into the Silicon under tightly controlled conditions of temperature and voltage bias, to neutralize the excess positive charges in the crystal creating, in a relatively simple way, an electrically pure diode that would be otherwise be very difficult to obtain.

 

Field Effect Transistor

The FET is positioned directly behind the crystal and makes electrical contact to the back face of the crystal. The FET converts the current pulse to a voltage pulse representative of the original photon energy by acting as a voltage regulated capacitor.

 

Pre-amplifier

The pre-amp supplies power to the FET and also receives and amplifies the output signal of the FET. This output signal is then directed to a pulse processor and analyzer system.

 

The FET is positioned directly behind the crystal and makes electrical contact to the back face of the crystal. The FET converts the initial surge of current in the detector, which results from the absorption of a single photon, to a voltage pulse which can then be amplified. The magnitude of the voltage pulse is proportional to the number of electron-hole pairs created in the crystal as the X-Ray is absorbed, and therefore, remains proportional to X-Ray energy.